The present invention relates to a nonlinearity compensation circuit and method, a control circuit and method for a nonlinearity compensation circuit, and a recording and/or playback apparatus which use the same, and more particularly to a nonlinearity compensation circuit and method for compensating for the nonlinearity of a read signal read from a recording medium, a control circuit and method for controlling a compensation amount of the nonlinearity compensation circuit, and a recording and/or playback apparatus which uses the nonlinearity compensation circuit and the control circuit in a signal processing system for the read signal.
In recent years, attention has been focused on a signal processing system called PRML (Partial Response Maximum Likelihood) for a recording and/or playback apparatus such as a digital magnetic recording and/or playback apparatus or a digital optical disk apparatus. The PRML signal processing system is a technique which can raise the recording density to 1.2 to 1.5 times through signal processing without modifying an existing recording and/or playback system extensively.
In a recording and/or playback system of the PRML signal processing system, a magnetic head, an optical pickup or a like element is used as a reading member for reading recorded information from a recording medium. Recently, a magneto-resistive head which makes use of a magneto resistance effect such as an MR head or a GMR head is used frequently as a magnetic head. The reason for this is that the magneto resistive head is higher in playback sensitivity and more suitable for high density recording than a conventional head of the inductor type.
However, the magneto resistive head generates second order distortion from its characteristic. Accordingly, a playback waveform of the magneto resistive head exhibits vertical asymmetry, and this restricts the recording density. Particularly, the PRML signal processing system which positively makes use of waveform interference is influenced significantly by a vertically asymmetrical playback waveform and cannot perform equalization well. In order to eliminate the problem, a circuit which makes use of polygonal approximation to perform nonlinearity compensation (refer to, for example, Published Japanese Translation of PCT No. 507157/1999) and another circuit which uses a squaring unit to perform nonlinearity compensation (refer to, for example, Japanese Patent Laid-Open No. 134501/1997) have been proposed conventionally.
However, the former circuit is disadvantageous in that it exhibits significant residual distortion. Meanwhile, the latter circuit is disadvantageous in that the circuit configuration is complicated and newly generates large amounts of third order distortion. Further, a control circuit for a nonlinearity compensation circuit does not control the nonlinearity compensation circuit based on a clear criterion and does not achieve optimum control. Besides, since the control circuit is configured so as to utilize error information of an equalizer or the like, it involves a comparatively large loop and cannot be designed so as to have a high degree of stability.
It is an object of the present invention to provide a nonlinearity compensation circuit and method, a control circuit and method for a nonlinearity compensation circuit, and a recording and/or playback apparatus wherein second order distortion can be removed sufficiently with a simple circuit configuration and such ill effects as third order distortion do not occur.
In order to attain the object described above, according to an aspect of the present invention, there is provided a nonlinearity compensation circuit, comprising compensation means for compensating for the nonlinearity of an input signal in response to a control signal, and characteristic provision means for providing an input/output characteristic represented by a function of
y=(x+c)/(1+cx)
where x is the input signal, c is the control signal, and y is the output signal, and xe2x89xa1x, cxe2x89xa1xe2x89xa61.
With the nonlinearity compensation circuit, the nonlinearity of an input signal to the nonlinearity compensation circuit is compensated for in accordance with the input/output characteristic represented by the function given above. Therefore, the nonlinearity of the input signal can be removed simply and sufficiently. Besides, such nonlinearity compensation can be performed without ill effects such as third order distortion. As a result, where the nonlinearity compensation circuit is applied to a recording and/or playback apparatus which records information onto a recording medium, the recording density can be improved.
According to another aspect of the present invention, there is provided a control circuit for a nonlinearity compensation circuit which compensates for the nonlinearity of an input signal, comprising measurement means for measuring a first time and a second time within which the waveform of the input signal has a positive value and a negative value with respect to a reference level, respectively, and control means for controlling the nonlinearity compensation circuit based on a difference between the first time and the second time measured by the measurement means.
With the control circuit for a nonlinearity compensation circuit, the first and second times within which the waveform of the input signal has a positive value and a negative value with respect to a reference level, respectively, are measured, and the compensation amount for the nonlinearity compensation circuit is controlled based on a difference between the first and second times. Consequently, the distortion amount of the input signal can be grasped with a maximum sensitivity and can be compensated for well. Accordingly, a system can be constructed in a self-complete fashion. Besides, a loop can be formed compact, and consequently, the control circuit and hence the nonlinearity compensation circuit can operate stably and on the real-time basis when it is actually used.
The nonlinearity compensation circuit and the control circuit for a nonlinearity compensation circuit can be incorporated suitably in a recording and/or playback apparatus for reading recorded information from a recording medium such as a magnetic disk, a magnetic tape or an optical disk and used as a compensation circuit for compensating for the nonlinearity of a read signal read from the recording medium and a control circuit for controlling the nonlinearity compensation amount by the compensation circuit, respectively.
In particular, according to a further aspect of the present invention, there is provided a recording and/or playback apparatus, comprising reading means for reading recorded information from a recording medium, a nonlinearity compensation circuit for compensating for the nonlinearity of a read signal read by the reading means, and a control circuit for controlling a compensation amount of the nonlinearity compensation circuit, the control circuit including measurement means for measuring a first time and a second time within which the waveform of the read signal has a positive value and a negative value with respect to a reference level, respectively, and control means for controlling the nonlinearity compensation circuit based on a difference between the first time and the second time measured by the measurement means.
With the recording and/or playback apparatus, the first and second times within which the waveform of the read signal has a positive value and a negative value with respect to a reference level, respectively, are measured, and the compensation amount for the nonlinearity compensation circuit is controlled based on a difference between the first and second times. Consequently, the distortion amount of the read signal can be grasped with a maximum sensitivity and can be compensated for well. Accordingly, a system can be constructed in a self-complete fashion. Besides, a loop can be formed compact, and consequently, the recording and/or playback apparatus can operate stably and on the real-time basis when it is actually used.
Where the nonlinearity compensation circuit is incorporated in the recording and/or playback apparatus, the nonlinearity of the read signal is compensated for in accordance with the input/output characteristic represented by the function given hereinabove. Therefore, the nonlinearity of the input signal can be removed simply and sufficiently. Besides, such nonlinearity compensation can be performed without ill effects such as third order distortion. As a result, the recording density can be improved.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference symbols.